A new flow-through micro-fluorometer with a working volume of less than 250 nL has been developed. The cuvette is closely coupled to the photomultiplier detector to insure efficient collection of fluorescence. Excitation energy is brought to the cuvet via a low-loss optical fiber; the light source is a 100 watt mercury arc. Tests have demonstrated the fluorometer's ability to measure concentrations of the fluorescein below 10 to the minus nine M/L and total amounts of less than 10 to the minus fourteen moles injected into a flowing stream of buffer. When used as part of a continuous flow analysis instrument the fluorometer should be valuable for femtomole level analysis of amino acids and enzyme substrates.